I've heard back from Fastly!

Specific to this particular incident, they've identified a few places where their own internal procedures fell short and they've rectified them. Specifically:

1. Their ticketing software saw the notifications from the Russian government/ISP as spam, most likely due to the Cryllic character set. This lead them to miss seeing the reports initially until later. They've resolved this by whitelisting those domains.
2. They notified the customers and then told the Russian government/ISP that the owner of the content as been notified. Instead they are going to ensure that the content in the future.

In the long term they are evaluating their own policies for how they host customer sites which allow user uploaded content (since those types of sites are the most likely to have these kinds of issues) and determining if it makes sense for them to require dedicated IP addresses for those customers.

For now I think Fastly has sufficiently handled the issue to not require some sort of backup system to need to be put in place. They are going to let me know how they are going to handle it long term and what, if any changes, we can make in our use of their service to help isolate from those kinds of issues.